An axial flow rotary machine, such as a gas turbine engine for an aircraft, has a compression section, a combustion section and a turbine section. In typical fixed-wing aircraft, the engine is mounted in a housing attached to the wing of the aircraft. The housing is commonly referred to as a nacelle. The nacelle both supports and positions the engine with respect to the aircraft. An annular primary flow path for working medium gases extends axially through the sections of the engine. In aircraft installations, the compression section commonly includes a fan section having a bypass duct. The bypass duct provides an annular flow path for secondary working medium gases which extends rearwardly about the primary flow path.
A fan rotor assembly in the fan section includes an array of fan blades which extend outwardly across the primary and secondary flow paths. A plurality of fan exit guide vanes are disposed downstream of the fan blades in the fan duct to receive the relatively cool working medium gases of the secondary flow path. A plurality of struts are typically disposed downstream of the fan exit guide vanes to support the stator structure and to transmit loads from the engine to its supporting structure.
During operation, working medium gases are drawn along the primary flow path into the compression section. The gases are passed through several stages of compression, causing the temperature and the pressure of the gases to rise. The gases are mixed with fuel in the combustion section and burned to form hot pressurized gases. These gases are a source of energy to the engine and are expanded through the turbine section to produce work. A portion of this work is transferred to the compression section to drive the fan rotor assembly and its fan blades about an axis of rotation.
The working medium gases in the fan duct have a mass flow which is six to eight times the mass flow in the primary flow, but with a relatively small pressure rise and a modest temperature rise.
Various components in the engine generate heat such as an electrical generator or an oil system for providing lubricating fluid to rotating components in the engine. Oil or another liquid medium is used to carry away the heat is discharged to maintain operative temperatures of these components within acceptable limits to cooling air in the fan duct or another acceptable heat sink, such as fuel for the engine.
One construction using lubricating oil as a means for removing heat and rejecting it to heat exchangers is shown in U.S. Pat. No. 4,151,710, entitled "Lubrication Cooling System For Aircraft Engine Accessory", issued to Griffin et al. The heat is rejected primarily to a heat exchanger extending into the secondary flow path of the engine and secondarily to a heat exchanger in communication with fuel being flowed to the combustion chamber.
Another example of a cooling system is shown in U.S. Pat. No. 4,474,001, entitled "Cooling System For The Electrical Generator Of A Turbofan Gas Turbine Engine", issued to Griffin et al. In the second Griffin reference, this cooling system rejects excess heat to the engine fuel through a primary heat exchanger and at low fuel rates supplementary rejects heat to fan air flowed from the working medium flow path to a secondary heat exchanger which is located remotely from the fan duct. A valve is used to turn on and off the flow to the fan air (secondary) heat exchanger as required under operative conditions of the engine.
Still another approach is to provide a heat exchanger disposed in a compartment of the nacelle which receives air from two sources: a pressurized compartment in flow communication with the compressor of the engine at low power; and, fan air from the fan bypass duct at high power. Cooling air is flowed from these locations to the heat exchanger and dumped overboard. Valves are required to interrupt the flow from the pressurized compartment at high power and to interrupt the flow from the fan duct at lower power.
Another approach is to provide a flow path to a heat exchanger in a core compartment which extends from an inlet in the fan duct to an outlet at a downstream location in the fan duct. The outlet is spaced downstream a significant distance (several feet) such that the outlet is at a location having a lower static pressure than the inlet to the exchanger flow path. The inlet and outlet are spaced apart by this distance to avoid regions of the fan duct that have the same static pressure, which creates an adverse static pressure gradient (zero or slightly negative) between the inlet and outlet.
The inlet protrudes into the fan duct such that it is spaced from the inner wall and the outer wall. The inlet faces the oncoming flow and drives cooling air through the flow path to the heat exchanger because of the difference in static pressure between the inlet and the outlet. The inlet structure is exposed to foreign object damage from ice and other debris which is ingested into the engine and centrifugal away from the inner wall to the interior of the fan duct. Such debris impacts the inlet to the heat exchanger and may be carried downstream to the heat exchanger where the debris may strike and block the heat exchanger.
As the cooling air is passed through the heat exchanger, the cooling air receives heat from components that are cooled by the heat exchanger. The heated air is discharged into the fan duct at the downstream location. The discharge temperature of the cooling air from the outlet of the flow path for the heat exchanger may approach unacceptable levels for the adjacent structure. This results from the heat load and level of cooling flow even though the distance between the inlet and outlet creates a difference in static pressure. As a result, a metal shield may be installed downstream of the exhaust. The shield is heated, and material radially inwardly of the shield, such as composite structures, are protected from the hot exhaust.
The above art notwithstanding, scientists and engineers working under the direction of applicant's assignee have sought to develop proof cooling systems which avoid complex valving, adverse affects on the efficiency of the operating engine.